The siren's pitch of a vehicle become higher as it
approaches to you and become lower as it passes your
position. This effect is known as the Doppler's
effect of sound.

It
is associated with the wave nature of light. As the
vehicle approaches, the sound waves from its siren
are compressed towards the observer, which increases
the siren's pitch. As the vehicle recedes, the
sounds waves are stretched relative to the observer,
which decreases the siren's pitch.